x86/uaccess: Document copy_from_user_nmi()

#include <asm/tlbflush.h>

/*

 * We rely on the nested NMI work to allow atomic faults from the NMI path; the

 * nested NMI paths are careful to preserve CR2.

/**

 * copy_from_user_nmi - NMI safe copy from user

 * @to:		Pointer to the destination buffer

 * @from:	Pointer to a user space address of the current task

 * @n:		Number of bytes to copy

 *

 * Returns: The number of not copied bytes. 0 is success, i.e. all bytes copied

 *

 * Contrary to other copy_from_user() variants this function can be called

 * from NMI context. Despite the name it is not restricted to be called

 * from NMI context. It is safe to be called from any other context as

 * well. It disables pagefaults across the copy which means a fault will

 * abort the copy.

 *

 * For NMI context invocations this relies on the nested NMI work to allow

 * atomic faults from the NMI path; the nested NMI paths are careful to

 * preserve CR2.

 */

unsigned long

copy_from_user_nmi(void *to, const void __user *from, unsigned long n)

	/*

	 * Even though this function is typically called from NMI/IRQ context

	 * disable pagefaults so that its behaviour is consistent even when

	 * called form other contexts.

	 * called from other contexts.

	 */

	pagefault_disable();

	ret = __copy_from_user_inatomic(to, from, n);